1. Technical Field
This invention relates to an apparatus and method for heating materials as part of a chemical process. More specifically, the invention is directed to kilns adapted for producing end or intermediate products by heating initial materials without direct contact of the initial materials by combustion products.
2. Statement of the Art
Structures and methods for controlled heating of initial process materials for purposes of producing intermediate or end products are well known. Such structures, whether of a static-type construction or alternatively of a dynamic construction, have been described in the literature. Fischer described both vertical shaft retorts and rotary kiln retorts and rotary kiln retorts that were operated in Germany prior to 1925. Phumpherton vertical retorts were operated in Scotland during the time period of 1894 to 1938. In Estonia, Davidson type rotary kilns were successfully operated from 1931 to 1961. In each of these instances, the use of these kilns was subsequently abandoned in view of low throughput rates, i.e., routineers were unable to achieve sufficiently high material throughput rates to meet desired production rates.
Tosco rotary retorts, developed in the U.S. prior to 1980, were indirectly heated by cycling hot alumina balls through a horizontal rotary pyrolysis reactor. The balls were directly heated in an external combustor. Galoter retorts, which have operated commercially in Estonia since 1963, employ a rotary reactor and a heating procedure similar to the Tosco retorts except spent shale is combusted and heated in an external fluidized bed and recycled through a horizontal rotary reactor. The heating procedure of the more recent ATP design is similar to the Galoter design except the pyrolysis reactor and shale combustor are connected by an extension of the cylindrical walls of the rotary pyrolysis reactor. All of these retort designs are directed to processing fine particles.
Oil producing pyrolysis reactions of coal, oil shales or other hydrocarbon materials typically occur when they are heated to temperatures of 400° C. to 500° C. Oil yields can be reduced by overheating the solid materials or the pyrolysis vapors. In a 1925 publication Franz Fischer indicated that in order to produce oil by the carbonization of coal, peat or oil shale, it is necessary to reach the decomposition temperature (400° C. to 500° C.) of all the constituents capable of forming oils. Fischer determined that it was advantageous not to raise the temperature beyond that absolutely required, so as to avoid any secondary superheating of the oil vapors, and even to cool the vapors as rapidly as possible below 300° C. He further posited that prolonged heating to higher temperatures would lead to the decomposition of valuable constituents with formation of gas and a consequent diminution of the oil yield.
A further description of the importance of temperature control is provided in U.S. Pat. No. 4,116,810, issued in 1978 and entitled “Indirect Heating Pyrolysis of Oil Shale.” The '810 patent teaches that a temperature on the order of slightly over 900° F. (482° C.) is necessary to convert the organic material of the rock to oil. As the temperature of pyrolysis increases, however, considerable differences may occur in the shale oil. For economic considerations, however, it would appear to be desirable to maintain pyrolysis temperature as low as necessary to produce oil and to convert substantially all of the organic material to oil. The lower temperature, also, reduces the inorganic carbonate degradation.
Further disclosure of alternative kiln constructions are disclosed in U.S. Pat. No. 5,068,010; U.S. Pat. No. 5,662,052; U.S. Pat. No. 2,723,226; U.S. Pat. No. 4,439,209; U.S. Pat. No. 5,312,599; U.S. Pat. No. 5,308,596; U.S. Pat. No. 5,393,225 and U.S. Pat. No. 4,181,491.